This invention relates generally to amplitude modulation apparatus and methods and more particularly, but not by way of limitation, to apparatus and methods for aiding deaf people to detect acoustical signals.
In the basic process of conventional amplitude modulation, the amplitude of a carrier signal is modified by a modulating signal to produce a modulated signal. The carrier signal is often a sine wave of constant amplitude whose frequency is much higher than that of the modulating signal. This type of carrier signal can be mathematically represented by the expression A.sub.c COS.omega..sub.c t, where A.sub.c represents the amplitude of the carrier signal and .omega..sub.c represents the radian frequency of the carrier signal. The modulating signal, on the other hand, is usually a complex waveform, such as the output of a microphone which is detecting speech or music. For the following mathematical analysis, we can express the modulating signal by the expression V(t) which indicates that the instantaneous value of the modulating signal is a function of time in some undisclosed relationship.
In a conventional amplitude modulation apparatus, a mathematical product of the carrier signal and the modulating signal is formed. For the aforementioned signals, this product is expressed M(t)=[1+kV(t)]A.sub.c Cos.omega..sub.c t. That is, the amplitude modulation apparatus takes a portion k of the modulating signal V(t), adds it to a constant (1 in this case), then multiplies the sum by the mathematical representation of the carrier signal. This produces the modulated signal, M(t). The quantity [1+kV(t)] is called the modulating function, and k is called the modulation factor.
It is apparent that if the modulating function is zero, M(t) will also be zero. The modulating function goes to zero whenever the quantity kV(t) goes to -1. When this occurs at the most negative values of V(t), the carrier signal is said to be 100% modulated.
If the quantity kV(t) becomes more negative than -1, thereby making the modulating function negative, a condition called "overmodulation" occurs whereby the modulated signal, M(t), becomes negative. When M(t) becomes negative, the actual modulated signal experiences a 180.degree. phase shift because -A.sub.c Cos.omega..sub.c t=A.sub.c Cos(.omega..sub.c t=.pi.). Such a phase shift introduces distortion into the modulated signal. This distortion is manifested in the form of additional frequency components which were not a part of the original modulating signal V(t). The severity of the distortion is determined by the extent of overmodulation.
From this brief review of the basics of conventional amplitude modulation, it is apparent that any carrier signal of any chosen amplitude will be 100% modulated by a modulating signal if the reciprocal of k equals the absolute value of V(t). The carrier signal will be overmodulated if the reciprocal of k is less than the absolute value of V(t). Therefore, overmodulation can be prevented by limiting the magnitude of V(t) so that its absolute value is not greater than the reciprocal of k; however, this limitation also prevents the carrier signal from being modulated to more than twice its quiescent, or unmodulated, amplitude. For example, if V(t)=.+-.3 volts zero-to-peak and k=1/3, this assures that kV(t) will not go more negative than -1, thereby insuring that overmodulation will not occur. However, this also limits the magnitude of the modulating function to two when V(t) reaches its maximum positive peak, thereby limiting the upward modulation of the carrier signal to only twice its quiescent value.
From this example it is apparent that ordinarily a carrier signal can be modulated-up to not more than twice its quiescent value if overmodulation is to be avoided during the negative-going portion of the modulating signal. This is true as long as the modulation process is symmetrical and in accordance with the aforementioned equation for M(t). For this equation to be valid, the complex modulating signal V(t) must have no direct current bias. That is, it must be balanced about zero volts and the area under the curve representing its positive-going portion must equal the area under the curve representing its negative-going portion. Additionally, the signal V(t) must have peak values which are equal in both the positive and negative directions if the modulation-up magnitude is to equal the modulation-down magnitude.
If it is desirable to achieve asymmetrical modulation, then limiting of either the positive-going excursions or the negative-going excursions of the modulating signal can be effected. However, care must be exercised in the method used to limit the excursions of the modulating signal because any changes to the modulating signal will result in distortions. Certain distortions can render the subsequent modulated signal unacceptable.
Amplitude modulation is, of course, well known and its uses are also well known. One of these uses is to assist people who have hearing impediments which prevent them from properly responding to acoustical signals in the same manner as those who have normal hearing. For example, it has been discovered that a person with a severe hearing problem, such as substantially total hearing loss, can be aided in properly responding to acoustical signals through the use of a cochlear implant device which is connected to the output of an amplitude modulation apparatus. An example of a cochlear implant device is disclosed in U.S. Pat. No. 4,352,960.
In the case of the cochlear implant application, it is necessary to produce a suitable amplitude modulated signal comprising a carrier signal modulated by a microphone signal. The microphone signal is an electrical signal representative of ambient sounds, such as the voice of one speaking to the person having the cochlear implant device. For this usage, it is desirable that the amplitude of the modulated signal be maintained between an upper limit and a lower limit. The specific upper and lower limits vary for each individual, but preferably the limits range from the threshold of perception of the individual up to the discomfort level of the person. It has been found that this range is typically between approximately 2 volts zero-to-peak and 10 volts zero-to-peak. Because this exemplary upper limit is more than twice the exemplary lower limit, it is apparent that an amplitude modulation apparatus and method which are able to vary a carrier signal by more than the basic factor of two, but which do not overmodulate, are required.
It is a further need that in the absence of a modulating signal (e.g., when there are no ambient sounds), the modulated signal which drives the cochlear implant device and which is essentially the carrier signal with little or no modulation remain at or just below the person's lower limit so that when any sound is detected, the carrier signal will be modulated to an amplitude above the perception threshold. However, this need is also not achievable by the conventional amplitude modulation operation because in the conventional amplitude modulation operation the quiescent level is at the median value between the upper and lower modulation limits rather than at the lower limit.
Therefore, in view of the shortcomings of the conventional amplitude modulation operation, there is needed an apparatus and method whereby a carrier signal can be modulated up from a quiescent value but not significantly modulated down so that the output of the modulation circuit or process is maintained at or near the quiescent value of the carrier signal when no or little modulation of the carrier signal occurs. There is also the need for such apparatus and method to be able to modulate the carrier signal up by more than the conventional factor of two, but without overmodulating the carrier signal, so that the full dynamic range of the person's auditory perception can be utilized. By meeting these needs, an amplitude modulation auditory stimulation apparatus having a quiescent unmodulated output voltage set at or just below the user's perception threshold and further having a modulation-up factor calibrated so that the modulated output voltage does not exceed the user's upper tolerance limit under any conditions of ambient sound can be achieved.
In addition to the foregoing needs which have been identified specifically with reference to people with hearing impediments, there is generally a need in the field of radio communications for an apparatus and a method for producing large amplitude variations on a carrier signal without producing significant distortion arising from overmodulation. The fulfillment of the need would enable a strong signal to be transmitted without requiring a high level of average power. For example, a small battery-operated transceiver could be provided, which transceiver would conserve the battery power and yet would send strong signals receivable at distances previously attainable only by higher-powered transmitters. Such apparatus and method provided by the fulfillment of this need would possibly have military, citizens band, and commercial radio applications.
Although the basic amplitude modulation device and method are known, as are amplitude modulation devices and methods which are to aid the hearing impaired, these prior devices and methods do not meet the aforementioned needs.